Fibre Metal Laminates (FMLs) have been widely utilised in aircraft structures for their specific strength, durability and damage tolerance. Most aircraft structures, due to their size, require joining, and FMLs allow for a special type of joint called the splice, a unique but a lesser known joint. The bulk of research into the splice joint has been conducted in the early 2000s for the A380 program, where a specific type of splice has been implemented, the overlap splice. While it is a proven design, it was expected that other splice designs could be as good if not better than the overlap splice. Further, the pre-existing design guidelines on the splices were not suited for detailed design that would target specific mechanical strengths of the joint, like durability and damage tolerance. Therefore, this research focused on design iterations of the two splice types, the butt splice and the overlap splice, that would provide insights into splice design guidelines against fatigue and demonstrate which splice configuration is a superior joint. An extension of this research focuses on identification of location and lifetime to damage initiation within the splice using analytical numerical methods, which was previously done on uninterrupted FMLs, but not on spliced FMLs. Research comprises of a multi-disciplinary approach, combining Finite Element Method (FEM), numerical predictive modelling and experimental investigations that then also doubled as means of validation of the models built. Design iterations were based on the influence that the parameters, labeled a through e, within the splice joint have on the overall stress field within the joint and how that affects the joint’s fatigue life to damage initiation, Ni, a fatigue performance indicator of the study. It has been discovered that changing the tolerances within the splices provides little influence on the stress field within the splice, which resulted in several iterations of the splice design approaching smaller and more lightweight joint alterations. The designs showed excellent durability characteristics when compared to the Limit of Validity (LOV) cycles set by the aviation authorities, such as EASA and FAA. The Finite Element (FE) model was able to accurately depict the stress field within the splice, validated by the experimental data through strain fields captured using Digital Image Correlation (DIC) technique, and consequently accurately pointed to the location of damage initiation, which in all cases was the outer-most overlap on the flush side of the joint. Along with the adapted predictive numerical model it was possible to predict the damage initiation lives in the spliced specimens with a blunt notch. The predictions in the samples without the notch resulted in far lesser agreement with experiments. This was likely caused by to the limitations of the model, which only took into account quasi-static loading without damage and is highly dependent on the reference data used. It was concluded that splices can be designed smaller and lighter than previously done due to the tighter tolerances allowed within the splice. Overlaps of 5 mm in both the unnotched butt splice and the unnotched overlap splice, however, resulted in an alternative damage progression mode, specifically a complete delamination of the external overlap rather than metal fatigue cracking. This is attributed to the rising average shear stress in the adhesive. Regardless, the smallest and lightest iterations of the butt splice and the overlap splice with 5 mm overlaps and gaps between aluminium interruptions showed excellent durability and the design iterations were found to affect the damage initiation life very little, considering that fatigue damage initiation is a subject to scatter. It is hard to draw a concrete conclusion on the damage tolerance of the updated designs due to alternative damage modes and lack of research thereof. While both splice types were deemed to be successful in their role of a joining structure, the butt splice was concluded to be superior to the overlap splice when it came to fatigue performance, consistently developing visible damage later than the overlap splice. This gap in fatigue performance is expected to expand if thicker layers or larger number of layers are to be considered. This is because of the secondary bending which occurred in the joint, more so for the overlap splice than the butt splice judging from FEA results and experiments. It is recommended that a more extensive FE model is developed using the model built in this research as a foundation. The current model could be expanded in several ways, such as simulations of damage progression, individual modelling of the fibre layers, fibre failure models, and a curing simulation. This will improve the validity of the methodology, specifically concerning the unnotched splice specimens, and improve the predictions of not only initiation lives, but also failure lives.

During this research, a new approach to predict impact damage was evaluated using the PAL-V Liberty’s propeller blade as the test subject. The purpose of this research was to provide a simple method for impact damage prediction which will support the certification activities of the PAL-V propeller. This method involves conducting a threat assessment, a Probability of Detection analysis, executing ASTM D7136 impact and quasi-static tests on coupon samples and conducting quasi-static tests on the propeller blade itself. The results from these tests were used to create a two-mass model which can predict the contact force during a propeller impact event. This model was validated using impact test on the propeller blade. The study conducted a series of ASTM D7136 impact tests and quasi-static tests on coupons, revealing that the coupons exhibited greater load-bearing capacity under impact loading compared to quasi-static loading. These tests also showed that the coupons have an increased stiffness when subjected to impact loading, requiring a multiplication factor of 1.47 to align their stiffness with quasi-static conditions. The research also involved determining boundary conditions for propeller tests, which led to the development of a custom clamping mechanism. During the quasi-static tests, the differences in stiffness between coupon and propeller tests were identified. These coefficients served as the basis for a predictive model involving a two-mass, spring and damper system, with parameters derived from coupon tests and the propeller quasi-static tests. After performing the validation impact tests on the propeller blade, it was determined that this model successfully predicted contact forces within 13% of actual test values. The research demonstrated the potential to predict impact energy requirements for barely visible impact damage, without the use of FEA. The model’s accuracy, while it can be improved, is sufficient to predict impact events effectively, offering a simpler alternative to complex FEA models.

Efficient maintenance scheduling is a critical objective for aircraft carriers and Maintenance Repair and Overhaul Organizations (MROs) to minimize aircraft downtime. While predictive maintenance models have improved, accurately identifying materials, especially spare parts, for specific maintenance events remains challenging. This paper combines the challenges of identifying spare parts by MROs and aftermarket distributor demand models by developing a robust prediction model (SPSO-CM) to forecast subsequent customer-specific purchases of maintenance planning document (MPD) related spare parts, considering technical documentation and previous sales records. The proposed architecture employs a gradient-boosting algorithm with numerous data mining improvement techniques to predict the likelihood of a subsequent spare part purchase from a customer. A k-means clustering algorithm is used to group spare parts with similar characteristics, as certain specific spare part properties significantly influence demand prediction models. A unique feature selector and nested group K-fold TimeSeriesplit cross-validation method were developed and incorporated into the Bayesian search space to optimize hyperparameters and improve performance. Two test cases were simulated, and the results demonstrated that SPSO-CM is more effective in forecasting proprietary parts and frequently purchased spare parts than those with extremely lumpy demand patterns. Two potential applications for an aftermarket distributor are discussed, one from a customer-level perspective and another at a larger supply-chain level, highlighting its promising capabilities.

To expand the use of additive manufacturing in aerospace towards more critical applications, it is required to design parts in a damage tolerant context. Therefore, the damage tolerance of additive manufactured multiple load path structures is assessed by analysing the fatigue life and damage propagation of components with increasing redundancy. An experimental approach is chosen, whereby specimens with 1, 9 and 81 parallel struts are tested. A decreased fatigue life is found for the specimens with more but thinner struts. This decrease is attributed to manufacturing related effects that occur upon producing smaller elements. The failure of the multiple load path structures showed a step-wise pattern. Due to this, the decreased variation in fatigue life and decreased sensitivity to initial damage, multiple load path structures are more damage tolerant. However, in design a balanced decision should be made upon applying these structures, due to the decreased fatigue life.

With a growing demand for longer blades in the wind turbine industry for higher rated power per turbine, a structurally sound blade-root connection is of commercial importance. Bushing connections have been a commercially favoured design in the past few years, replacing the commonly used T-bolt connection as the joining method of choice. The new design replaces the barrel nut in the T-bolt with an axial bushing that the bolt connects to and can be assembled with the laminate during the lay-up stage of the blade skin. It has been theorised that it can result in a reliable connection due to the elimination of laminate stress concentrations. However, literature outlining the performance of a blade-root connection with bushings is lacking in the current body of knowledge. While several patents for bushing designs exist, they don’t provide verifiable results on their efficacy due to trademark laws. The objective of this project is to design and conduct a numerical study of a blade-root connection with bushings with an aim to replace the T-bolt connection, along with providing evidence of the effect of various parameters on the structural performance of the blade root. The design is to be based on a Suzlon Energy-make blade with a pitch circle diameter of 3m and a blade length of 63m. The project has been planned in three phases: (1) Design of the root; (2) Validation of the design; (3) Comparative analysis. Modelling and FE analysis has been carried out in the ANSYS environment. Parameters of the bolted connection have been determined according to industry standards provided by VDI and GL. Design validation was conducted based on structural constraints; the key design constraint relevant to the blade-root as a sub-component of the wind turbine is the accumulated fatigue damage. For the final phase of the study, various parameters associated with the assembled root were identified and tested in iterations and their effect on the structural performance, weight, and cost of the assembly were studied. The results confirm the hypothesis of reduction of stress concentrations within the laminate; this eliminates several failure modes associated with composite laminates at the blade root. As is, the bushing connection can be considered a viable alternative to the T-bolt joint. Within the connection, higher absolute stresses and stress gradients were developed in the bolt joining the blade to the hub. Hence, this was the area of focus for fatigue damage evaluations. Conservative estimates of the accumulated damage show values well within the acceptable range. The connection has been designed keeping several concurrent variables in mind. Given the commercial applicability of the design, a rigid optmized design is not feasible due to unpredictable parameters like certification costs, total assembly times, and procurement costs. Therefore, an effort has been made to understand the effect of varying component parameters. The design lends itself to flexibility of dimensioning and material choice within the sub-components; parameters can be optimised according to cost, manufacturability, and performance requirements. While the base design configuration for the bushing connection is heavier than the T-bolt design, improved fatigue performance can be seen as a favourable trade-off.

The concept of lightweight design is driving future aircraft to exploit all the available strength of materials and further reduce the weight of an aircraft leading to lower fuel consumption and more sustainable aviation. Current designs introduce rivets and bolts to join the structure and simultaneously create holes in the pristine material, which is not wanted due to the local stress concentration. If the design wants to exploit all the strength of the material, stress concentration should be avoided using suitable joining technologies that don't require holes in the structure. This is possible with the adhesive bonding technology. However, many factors, such as the effects of manufacturing and impact-induced damage are still not fully understood. Thus, the damage tolerance of the design cannot be guaranteed. This results in conservatory safety factors being prescribed in the design process, which possibly reduces the exploitation of all available material strength. The occurrence of barely visible impact damage (BVID) in aircraft composite structures, especially in adhesive joints, is a serious issue that can jeopardise an aircraft's structural safety during operation. However, virtual testing and numerous numerical approaches allow high-fidelity simulation of damage initiation and propagation in adhesives and composite adherends to predict the residual strength of the bonding accurately. Although the development is fast, accurate impact simulation is still computationally very expensive and thus not applicable in industrial cases where behaviour needs to be analysed to assess the structures' design, maintenance and repair. This thesis investigated two topics that allow robust and accurate simulation methods to assess the effect of manufacturing and impact damage on the residual strength of bonded joints. The first topic is related to the development of the composite material damage model, where the multiscale material model is created by the use of a representative volume element (RVE). The second topic of simulation methodology is the development of the quasi-static simulation approach for the damage tolerance assessment of bonded joints. Modelling approaches to simulate residual strength are investigated through the finite element modelling of three groups (i) pristine, (ii) artificially damaged during manufacturing, and (iii) impacted bonded joints. In the numerical models, an approach based on the observation of the fracture surface of single-lap joints in different geometry and layup configuration is proposed in which the damage assessment focuses on the bondline area up to the first 0° ply. For the modelling of the damage in joints, two modelling techniques were studied, first removing elements in the damaged area and second detachment of the elements. Utilising those techniques, simplified approaches to model damage resulting from the impact were studied, with the modelling of impact damage as a hole, where all through-thickness elements are deleted and as delamination, where interlaminar cohesive zone elements are deleted. The developed multiscale material model allowed for an accurate representation of failure modes occurring in the composite material by the characterisation of elastic, damage and plasticity parameters of the fibre and matrix constituents in the homogenisation and inverse characterisation process. The results showed that the deletion of the elements can be used to represent defects and damage of different kinds in the composite bonded joints, both in the adhesive and adherend. The comparison of different representations of the impact damage in the single lap joint configuration revealed that the best prediction in terms of the ultimate load, failure mode and size of the model is obtained with the simplified representation as a single delamination positioned before the first 0° ply in the layup and in the studied adherend configuration that was between the first (45°) and second (0°) ply in the layup. The study ends with the conclusion that in different geometries of single lap joints and different damage types studied, a numerical analysis should focus on the region of the overlap edge and in the thickness direction from the bondline up to the first 0° ply in the lay-up. The results and numerical method developed during this thesis create a base for the further investigation of a variety of impact cases on bonded joints.

A Helically wrapped structure is a hollow pipe structure that is made from a metal strip and the overlapping interfaces are adhesively joined. It is yet unknown how to determine the quasi-static strength and predict the fatigue life of this structure. A method was developed to determine whether either the adherend or the adhesive is critical to determine the strength and fatigue life. A machine was made to create the helically wrapped structure and tested for validation. It was found that the adherend is critical instead of the adhesive.

This research project investigates the potential use of ultrasonic welding as a tool to repair impact damage inside thermoplastic composites to restore the compressive strength. Compared to the already existing method, hot-pressing, ultrasonic welding could offer faster processing times, no need for dedicated tooling, smaller and lighter machinery and a smaller (heat) affected zone. It was shown that ultrasonic welding is able to melt and re-consolidate delaminations, decreasing the damaged area. However, even when using the same welding parameters, the process showed large variations between the welds. These variations should be minimized for the process to be usable as a repair process. Compression after impact (CAI) testing showed that welded repairs are able to fully restore the CAI strength. The re-consolidated material inside the damaged area suppresses local buckling, which normally causes premature failure. However, it was also shown that repairs with little re-consolidated material can decrease the CAI strength.

This thesis aims to study the design and optimization of composite wing components for aircraft, focusing on the pivotal goal of reducing weight while preserving and enhancing structural performance. This is one of the goals of the COST Action joined in the span of the thesis: CA18203 - Optimising Design for Inspection (ODIN). Within this COST Action, a comparison between different Finite Element modeling predictions of the response of this structure subjected to 4-point loading has been presented by Bisagni et al. [1] before this thesis. The thesis, which corresponds to the optimization of this same structure, is the continuity of this work. The foundation of this thesis is built upon a literature study that investigates the optimization of composite wing structures. It explores diverse optimization formulations, including design variables, objectives, and constraints. Different methodologies for optimizing wing structures, like multi-objective and probabilistic methods, are studied. Various algorithms used to tackle these challenges are presented, shedding light on their advantages and drawbacks. Once a solid knowledge of wing structure optimization is built, preliminary analyses (study of coupon geometries, failure criteria, and the buckling behavior of various materials and geometries) form the basis for the upcoming optimization strategy. These preliminary investigations refine our understanding and guide the selection of optimal constraints, such as failure criteria. Based on these preliminary analyses and the literature study, the optimization strategy has been established. A new configuration with cut-outs is developed to explore new innovative designs. To solve the optimization problem, Genetic Algorithms, inspired by real-life behaviors and processes, emerge as the optimization algorithm of choice. These algorithms contribute to the development of design solutions that are better than traditional designs. The objective is to achieve weight reduction in the structure while considering its structural performance. Consequently, the fitness function, employed to assess candidate designs within the Genetic Algorithm, is based on three primary factors: weight, stiffness, and buckling behavior. Weight carries the most significant part of the fitness function, accounting for approximately 80% of the total value, whereas the remaining two factors contribute approximately 10% each. To evaluate these aspects, weight is determined through basic calculations, while stiffness and buckling behavior are assessed by simulating a 4-point bending test using Abaqus. The implementation of the Genetic Algorithm, adapted to our special case, allows us to showcase its effectiveness in achieving optimal designs. These optimal designs achieve remarkable weight reductions while maintaining great structural performance. Following 20 generations, each comprising 10 potential candidates, one of the two termination criteria is triggered: there is no improvement in the best solution for eight consecutive generations. The optimized solution obtained is 16.89 kg lighter compared to the baseline configuration studied by Bisagni et al. [1]. This represents a significant 25% reduction in structure weight. Concerning the structural performance, only a reduction of 1.5% and 0.25% is observed for the stiffness and buckling performance, respectively, which is acceptable regarding the weight reduction. Nonetheless, achieving such a reduction in wing weight may not be realistic as it was specifically tailored for the test case of this thesis, other loading scenarios need to be studied to assess its applicability. These outcomes underscore the potential of optimization techniques in aerospace engineering, paving the way for the development of lightweight and high-performing composite wing components.

Wind energy is a growing industry, and in an effort to reduce costs and increase turbine efficiency, rotor blades are becoming increasingly large in size. To facilitate this effort, the SmartBlades2 research project has designed, built, and tested a set of prototype research blades. As part of the SmartBlades2 project, high sensor density modal testing has been conducted on the research blades. The analysis of the modal tests showed good agreement of the global vibration modes with the finite element model predictions. However, the test analysis also identified low frequency vibration modes, referred to as breathing modes, which were not predicted by the finite element models. These vibration modes were found on all of the blades and are characterised by out-of-plane trailing edge panel motion. The objective of this thesis is to identify and predict the aforementioned breathing modes using finite element analysis. To achieve this, three model characteristics are analysed to determine their influence on the breathing mode prediction, namely, model topology, shell element configuration, and material properties. To characterise the affect of model topology, a cut section from the SmartBlades2 prototype blade is modelled with shell elements and continuum element glue joints. To validate the blade section model, a modal test is conducted which identifies breathing modes analogous to the full blade. Various topology features are investigated with the focus on the shell glue joints and spar web joints of the blade section. The analysis shows that while these changes significantly effect the mode shapes and frequencies, none of them predict the experimentally identified breathing modes. To investigate the source of this discrepancy, the modal behaviour of a sample plate structure with the same materials is used to remove the variability of topology. The effects of shell element size and configuration are analysed with mutual comparisons. The analysis shows that higher fidelity element configurations offer no advantage over linear shell elements for prediction of modal behaviour, while the element size shows higher sensitivity. Furthermore, the effects of material properties are examined using the sample plate, subject to modal and flexural tests. It is found that the specified properties are stiffer than measured, and new predictions of the properties are made which better fit the plates experimental results. Finally, the topology, element, and material investigations are then applied to an improved finite element model of the complete blade and correlated with the experimental modal tests. It is found that the improved blade model has closer correlation with the experimental modal tests for global modes, however is unable to predict the identified breathing modes for the blade. It is hypothesised that cause of this may relate to the connection of the spar web with the glue flanges.